The present invention relates to a mass spectrometer and a method of mass spectrometry.
Ion mobility spectrometry (“IMS”) is a well established analytical technique where ionic species are separated according to their ion mobility by subjecting the ions to a weak electric field in the presence of a buffer gas. A known ion mobility spectrometer comprises a linear tube filled with gas. A static homogeneous electric field is maintained along the length of the tube. Ions experience a force in one direction due to the electric field and an effective force in the opposite direction due to collisions with the buffer gas. To a first approximation, the equation of motion of an ion within the known ion mobility spectrometer can be written as:
                                                                        ⅆ                2                                            ⅆ                                  t                  2                                                      ⁢            x                    +                                    λ              m                        ⁢                          (                                                ⅆ                                      ⅆ                    t                                                  ⁢                x                            )                                      =                  E          ⁢                                          ⁢                      q            m                                              (        1        )            wherein t is time, x is the axial position along the length of the ion mobility spectrometer, λ is the drag coefficient, m is the mass of the ion, E is the electric field strength and q is the charge on the ion.
In this regime ions quickly reach a steady state velocity and the average acceleration becomes zero. Under these conditions the above equation of motion reduces to:
                                          ⅆ                          ⅆ              t                                ⁢          x                =                  E          ⁢                                          ⁢                      q            λ                                              (        2        )            
In Eqn. 2, the ratio q/λ is termed the ion mobility K. Ions having a relatively low ion mobility reach a lower steady state velocity than ions having a relatively high ion mobility and thus take longer to traverse the length of the ion mobility spectrometer.
Another known ion mobility spectrometer comprises a series of ring electrodes. A two-phase RF voltage is applied to the ring electrodes in order to create a radial pseudo-potential well which acts to confine ions radially within the ion mobility spectrometer. A series of pulses or transient DC voltages are applied to the electrodes and are translated along the length of the ion mobility spectrometer. The ability of an ion to keep up with the series of DC pulses which are translated along the length of the ion mobility spectrometer is a function of the mobility of the ion. Relatively low mobility ions are overtaken by the transient DC voltage more often than ions having a relatively high mobility. As a result, ions having a relatively high ion mobility are preferentially urged along the length of the ion mobility spectrometer whereas ions having a relatively low ion mobility take a relatively long time to traverse the length of the ion mobility spectrometer.
It is known to couple an ion mobility spectrometer to either a quadrupole rod set mass analyser or an orthogonal acceleration Time of Flight mass analyser. The separating characteristics of the known ion mobility spectrometer enable the duty cycle and sensitivity of either the quadrupole rod set mass analyser or the Time of Flight mass analyser to be improved. Furthermore, determining the drift time of ions through the known ion mobility spectrometer also reveals structural information about the ions.
It is desired to provide an improved mass spectrometer.